Introduction

We like to think of the universe as a place filled with life. We have been taught by movies, television shows, scientists and the media that there are myriad planets out there harboring life. But discovering intelligent life is what we are really excited about. Finding microbes, plants, or little furry rodents running about on another planet would certainly be amazing, but to find an alien civilization with culture, art, technology, and the ability to communicate their knowledge and perceptions to us would truly be one of the most fulfilling achievements of humankind. We would know we are not alone in the universe.

But is this notion of a universe filled with alien civilizations realistic, or is it just wishful thinking? There are an estimated septillion stars in the universe. That is 10 followed by 24 zeroes. That is a lot of stars and a lot of planets orbiting them. But there are many specific conditions which must be met to allow intelligent life to develop. Each condition alone may seem like it is not too restrictive, but when considering they all must be satisfied together, perhaps that combination is one chance in a septillion. And we would be that one chance. If we are the only intelligent life in the universe, it would seem to us that intelligent life should flourish in the cosmos, simply because we are here. It is natural to assume it exists elsewhere too. But it is, perhaps, only an illusion.

What follows are some of the many conditions that must be met for intelligent life to exist on any given planet.

Habitable Zone

The habitable zone about a star system, where temperatures for life on a planet will be just right. | Source

Right Distance From a Star

Water is viewed by scientists as a requisite for life. It is the chief medium through which all life’s basic building blocks, the cells, take in what is needed and expels what is not. It is not surprising, then, that scientists consider conditions suitable for water as a top priority when seeking for the existence of life beyond Earth. One such condition is called the “habitable zone”.

The habitable zone of a star system is the distance from a star that a planet must orbit in order for liquid water to exist. This distance is a range, a belt of certain thickness that circles a star. The less dense a star is, the closer to the star the region lies and the narrower it becomes. At distances outside of the habitable zone, conditions become too extreme to maintain liquid water, and therefore to sustain life.

A planet which orbits its star too closely will suffer the effects of the star’s intense infrared radiation. The planet’s atmosphere would trap so much of the heat that all its water would boil away. For a planet orbiting too far from a star, such little heat reaches the planet that its greenhouse gases cannot trap enough of it and all the water freezes. In both cases cells, and therefore life, would not have water as a medium in which to thrive.

Molten Interior

The heat and composition of a molten core will force its contents up to the crust of the planet, where it breaks free to the surface. This out-gassing will help create an atmosphere with such components as water vapor, carbon dioxide, nitrogen, and methane. The much-needed oxygen that supports animal life comes later from plants once they have evolved.

A planet’s magnetic field shields it from cosmic radiation. A liquid metallic core creates a magnetosphere which protects life from solar wind, flares and radiation from space. Without this, irradiation would kill life and solar winds would sweep the atmosphere away.

A molten core also creates plate tectonics. On Earth, the shifting plates pushed the crust up so that much of the surface stood above water to become land. Without the rumpling of the surface caused by the molten core, the earth would be covered entirely by an ocean. Life can arise in an ocean, but you probably would not find advanced civilizations there with no land to evolve on. After all, where would the opera perform?

Current theories suggest that a small planet collided with Earth to form the Moon. | Source

Twin Planet

Earth and its moon are essentially a twin planet. Whereas all the other planets’ moons are tiny fractions of their size, our moon is a quarter the size of Earth. Put them together, and the Moon looks like Earth’s little brother, whereas the other planets’ moons look like they might be their pet ants.

Because of the Moon’s great mass and proximity to Earth, its gravity helps stabilize Earth’s rotation. The earth would wobble radically about its axis on its own, but the Moon greatly reduces the wobble to a negligible amount.

The Moon’s gravity also gives Earth’s rotation the right speed and tilt to keep conditions constant enough to develop and support life. Without the Moon to stabilize the earth’s axis, the axis would at times point towards the Sun, and at other times the equator would point towards the Sun, causing wild temperature variations across the planet and shifting ice caps.

Mass Extinctions, the biggest "disasters" in history, happening at the right times and in the right amounts may have actually promoted the development of intelligent life. | Source

Timing of Events

The evolution of intelligence on Earth has depended greatly on many specific circumstances occurring over vast periods of time.

The great oxidation event, which took place when some bacteria began to photosynthesize, filled the atmosphere with the process’s waste product, oxygen. Thus breathable air was formed.

Twice in its history, Earth has frozen over completely. These times of “Snowball Earth” may have brought about the first complex animals.

Periods of extreme global cooling and an asteroid strike have caused mass extinctions which allowed for the evolution of more adaptable species and the proliferation of mammals, which eventually led to primates and humans. It was rather difficult for diminutive rodents to get a firm foothold on evolution with all those dinosaurs running around. A little help from a big rock crashing through the atmosphere goes a long way to clear the slate.

Orbit a Star That Is the Right Size

Complex life on a planet relies on dependable energy from its star. In order for something as complex as intelligent life to evolve, that star has to produce energy at a consistent rate for billions of years. A deviation in energy output too far in either direction can be devastating. If the radiated heat goes too high, it can boil the surface of the planet and anything on it. If the star’s heat is too low, it will freeze any life on the planet out of existence.

Stars with masses above 1.5 times that of our sun die too quickly to allow the time for life to evolve to intelligence (we humans took over 3 billion years). Stars that are smaller than our sun have a greater chance of tidally locking a planet’s rotation, keeping the same side of the planet towards the star. The atmosphere is likely to disappear as its gases condense on the eternally cold side of the planet.

A gas giant forming in an early star system. | Source

Distant Massive Planets

The presence of two or more massive planets, or “gas giants”, in a star system tends to shield smaller inner planets from stray asteroids. In our solar system, their combined gravity and orbits flush many asteroids and comets into interstellar space, safely away from Earth. Too many asteroids or too big an asteroid colliding with Earth, and life would not stand a chance. But if a gas giant is too close, its great gravity will prevent a planet from even forming, which is how our asteroid belt came to be. So in order for a planet to enjoy the shielding effect of a massive planet and not become a stillbirth of little rocks itself, that massive planet had best orbit an appreciable distance away.

A supernova, the explosive death of a star. | Source

Not Orbit a Star That Is Too Close to a Cosmic Explosion

Supernovas, those spectacular explosions of dying stars, can cause equally spectacular destruction of life to nearby star systems. In our galaxy, supernovas occur once or twice every one hundred years. Any planet within fifty light years would have its ozone layer damaged by the radiation of the blast. Life on that planet would likely perish due to massive amounts of its own sun’s ultraviolet radiation bombarding it through the unprotected atmosphere.

Another kind of explosion, called a gamma ray burst, can be caused by a binary star system. These stars shoot out a narrow, but very powerful, beam of energy that could also destroy the ozone layer of any planet unfortunate enough to lie in its path, again resulting in loss of life. These bursts can be ozone-killers at least as far away as 7,500 light years.

Planet Not Be So Massive That It Becomes a Gas Giant

Many conditions of gas giants make intelligent life very problematic, if not impossible. Gas giants retain enormous amounts of hydrogen and helium in their atmosphere and have almost no water. Some gas giants have no solid core for complex life to form on, and any that have a distinct surface would be subject to atmospheric pressures a thousand times that on Earth. Floating life forms could exist in the upper atmosphere, but most likely could not persist because of the highly chaotic nature of the atmosphere which would drag anything down via convection currents into the fatal high-pressure low layers near the core.

Stability of the Star System

In the early days of our own solar system, the gas giants orbited much closer to the sun and with more erratic orbits, putting them dangerously close to the smaller inner planets. The danger came from all the asteroids, comets, and other space debris the giant planets tend to attract. With all these swirling, speeding projectiles constantly bombarding the inner planets, life would not have had a chance to evolve beyond the toughest buried bacteria. Such life-inhibiting conditions are probably common in star systems across the cosmos.

Source

Consistency of Temperatures on a Planet

In addition to the long-term constant heat output of the Sun, the earth has managed to sustain relatively constant temperatures on its own surface despite any other influences. The earth’s steady temperatures across very long periods of time are crucial to the development of anything as complex as intelligent life. When temperatures vary too much over time, only the simplest life forms can survive; complex life cannot withstand such fluctuations. It is truly remarkable to consider that life has existed here for over 3 billion years, with complex life extending back 500 million years, and in all that time the temperature of our planet has not veered off so far as to freeze or bake everything out of existence. Just a change in the global temperature by a hundred degrees, colder or hotter, for a few centuries – small amounts of temperature and time in this universe – and life would have been extinguished completely.

Poll: Prevalence of Intelligence in the Universe

How prevalent do you think intelligent life (civilization) is in the universe?

We are the only intelligent life in the universe.

There are just a few civilizations in the entire universe.

There is roughly one civilization per galaxy.

There are a few civilizations in each galaxy.

Every galaxy is teaming with intelligent life. (exempting chaotic galaxies)

Conclusion

Mathematically, the odds may be slim enough to present only one planet in the universe as statistically possible to support intelligent life. If there are a septillion planets, each of the preceding points would, on average, only have to be as improbable as 1 chance in 250 of occurring. If so, considering they all have to qualify together, the chance for intelligent life to arise in the universe is 1 in a septillion. That is, only one planet in all the universe might harbor intelligent life, that one planet being our beloved Earth, and that life being us. If we are the only intelligent beings in all this vast universe, we are more precious than anything. We owe it to ourselves and to the universe to perpetuate our existence, to explore as far as we can, and to seek the knowledge to understand the universe as deeply as possible.

Questions & Answers

Questions must be on-topic, written with proper grammar usage, and understandable to a wide audience.

Question:

Why would there be one civilization in an infinite universe?

Answer:

Because the universe is not infinite. And because all the improbabilities added up can result in only one civilization.

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